The power of a pressurized water nuclear reactor can be controlled by various means, including the control of the insertion or withdrawal of the control clusters, i.e. control rods of a neutron-absorbing material. These control clusters are arranged parallel to each other inside the reactor core, among the fuel elements.
It is highly important, when the reactor is operating, to control not only the power of the reactor but also the distribution of this power. In fact, a faulty distribution of the power can threaten the integrity of the fuel elements which can be subjected to an irreversible deterioration under the effect of the heat which they produce, if this quantity of heat is too great to be absorbed and carried away by the heat transfer fluid or if the circulation of the heat transfer fluid is inadequate. The two phenomena which must, above all, be avoided are, on the one hand, the boiling criticality of the pressurized water in contact with the fuel elements and, on the other hand, the melting of the fuel. The boiling criticality can lead to the formation of a film of steam along the fuel elements, this film prohibiting the high exchanges between the fuel element and the pressurized water, owing to a calefaction phenomenon. As for the melting of the fuel, this is due to an excessive evolution of power reflected in a local rise in temperature up to the melting point of the material; the melting is produced when the value of the power per unit length, or linear power, exceeds a certain threshold.
Both the abovementioned phenomena, which threaten the integrity of the fuel elements, can be due to a faulty position of the control clusters: an untimely fall of a control cluster which is unhooked from its control shaft, an unhooked cluster which remains completely inserted at the start-up of the reactor, or an offset of a cluster relative to the neighboring clusters.
It is known to locate the position of the shafts for control of the clusters but, in the case of a cluster which is unhooked from its control shaft, for example, the location of the position of the control shafts would not enable a fault to be detected.